(1) Field of the Invention
The present invention lies in the field of optimizing the performance of an aircraft, and more particularly of a rotary wing aircraft on takeoff.
The present invention relates to a method of determining the wind speed to be taken into account in order to optimize the takeoff weight of an aircraft, and also to a device for performing the method.
(2) Description of Related Art
The ability to optimize the performance of an aircraft is a crucial element for operators of any aircraft. The performance of an aircraft is strongly influenced by the speed of the aircraft relative to the surrounding air. For convenience, the term “air speed” is used below to designate the speed of an aircraft relative to the air. This speed is often referred to as “true” air speed or “TAS”.
Specifically, the power that is consumed by an aircraft, and in particular by a rotary wing aircraft, depends on its air speed, regardless of the stage of flight being performed by the aircraft. By way of example, Document EP 1 078 308 describes a limiting envelope for admissible air speeds of an aircraft that is converted into a limiting envelope of speeds relative to the ground by taking account of speed and wind direction.
The maximum weight of an aircraft, and in particular its takeoff weight is also a function of the air speed of the aircraft. In addition, in the particular situation of an aircraft taking off at zero speed relative to the ground, such as a rotary wing aircraft, the air speed of the aircraft corresponds to the wind speed at the aircraft.
Thus, the maximum takeoff weight of a rotary wing aircraft is defined firstly by the technical characteristics of the aircraft and secondly by the wind speed, and more particularly the speed of the longitudinal wind to which the aircraft is subjected. The term “longitudinal wind” is used to mean the projection of the wind onto the longitudinal direction of the aircraft. An aircraft has three preferred directions: a longitudinal direction, a transverse direction, and a vertical direction, thereby defining a local rectangular reference frame tied to the aircraft.
For example, for a rotary wing aircraft flying at high air speeds, typically faster than the optimum climb speed VOM of the aircraft, when the speed of the head wind to which the aircraft is subjected increases, the power needed by the aircraft to perform level flight also increases. This increase in air speed gives rise in particular to an increase in the aerodynamic drag to which the aircraft is subjected, thus requiring an increase in the power the aircraft needs to deliver.
In contrast, at low air speed, typically speeds that are less than or equal to the optimum climb speed VOM of the aircraft, when the head wind to which the aircraft is subjected increases, the power needed to enable the aircraft to perform level flight decreases. In this range of air speeds, the increase in air speed serves to generate an increase in the lift of the main rotor that is greater than the increase in the aerodynamic drag to which the aircraft is subjected, and it is thus favorable to flying the aircraft.
For an aircraft having a power plant delivering a given power, the maximum takeoff weight of the aircraft can vary depending on the speed of the head wind to which the aircraft is subjected during takeoff.
Aircraft are conventionally provided with an air data computer (ADC) that provides the pilot with an indication of the aircraft air speed. This air speed is equal and opposite to the wind speed when the aircraft is on the ground and stationary, in particular while waiting to take off. Such an aircraft data computer conventionally makes use of at least one Pitot tube measuring the total air pressure in the longitudinal direction of the aircraft, and a measurement inlet for measuring the static pressure of the air surrounding the aircraft.
Under such circumstances and by way of example, an air data computer having two measurement devices positioned in the longitudinal and transverse directions of the aircraft can supply the pilot of the aircraft with indications about the air speeds of the aircraft along a longitudinal component and along a transverse component, and thus an indication of the magnitude and an indication of the direction of the wind while the aircraft is stationary on the ground.
Nevertheless, the air speed supplied by a conventional air data computer is not always accurate and reliable.
That type of instrument is not capable of measuring wind speeds that are low of less than 30 knots (kt), with good stability and good accuracy. Furthermore, that type of instrument measures air speed in one direction only, such that the measurements for the magnitude and the direction of the wind when the aircraft is stationary on the ground can be inaccurate.
Furthermore, rotary wing aircraft have at least one main rotor that is driven in rotation about an axis that is substantially vertical and that serves to provide the aircraft with lift, and possibly also with propulsion. In addition, such rotary wing aircraft may also be provided with an anti-torque device comprising at least one auxiliary rotor driven about an axis of rotation that is substantially horizontal, such as a tail rotor.
Consequently, each main rotor and possibly a tail rotor produce respective washes resulting from their own rotation that can disturb the air stream surrounding the aircraft in the vicinity of its air pressure inlets. Measuring the air speed of the aircraft can be disturbed by the presence of each main rotor and possibly of a tail rotor, which effect can become even greater when the aircraft is stationary on the ground.
Consequently, measurements provided by conventional air data computers on board a rotary wing aircraft do not generally make it possible to obtain a measurement of wind speed that is accurate and of integrity, when the aircraft is stationary on the ground.
Furthermore, an aircraft's maximum authorized takeoff weight is considered to be a safety characteristic of the aircraft and the way it is determined is governed by regulations and requirements defined by various organizations such as the European Aviation Safety Agency (EASA) for Europe, for example.
In particular, in the EASA requirements, if a measurement of the head wind to which the aircraft is subjected is not available in a manner that is sufficiently accurate and of sufficient integrity, then a safety margin is applied to the speed of the head wind that is used for determining the maximum authorized takeoff weight of an aircraft. The safety margin is greater than or equal to 50% of the speed of the head wind measured in the proximity of the aircraft.
In practice, this safety margin is generally equal to 50% of the speed of the head wind measured in the proximity of the aircraft, the longitudinal wind speed that is used for determining the maximum authorized takeoff weight of an aircraft then being equal to that speed of the head wind observed in the proximity of the aircraft when divided by two. That observed speed of the head wind is generally taken from a weather observation report for the aerodrome from which the aircraft is to take off, which report is established prior to the aircraft taking off and is issued in regular manner by the aerodrome.
That observed head wind speed is measured on the runway or on the aerodrome from which the aircraft is to take off but not necessarily at the aircraft itself. Furthermore, that observed head wind speed is not measured at the time the aircraft takes off, but rather beforehand, and generally at regular intervals. By way of example, that observed head wind speed may have been measured more than 30 minutes (min) before the aircraft takes off.
Finally, when taking off from a non-prepared area, it can happen that mean wind information is not available for that area, but is available for an area that is some distance away.
Furthermore, when no mean wind measurement is available, the local wind speed is considered to be zero.
Such a measurement of the observed head wind does not correspond exactly to the conditions that are actually encountered by the aircraft when it takes off. Nevertheless, that 50% safety margin represents a solution that is reliable and safe, satisfying the requirements of regulations, in particular for rotary wing aircraft.
Nevertheless, nowadays there exist bidirectional anemometers and multidirectional anemometers that provide air speed measurements that are more reliable and that can also be of integrity. In particular, such bidirectional anemometers make it possible to define a longitudinal speed and a transverse speed for the air speed of the aircraft. Multidirectional anemometers are capable of defining the magnitude of the air speed and its direction in a local reference frame tied to the aircraft. Furthermore, such bidirectional and multidirectional anemometers are also capable of measuring low air speeds, down to values that are zero or negative.
For example, optical anemometers are known, such as light detection and ranging (LIDAR) anemometers that can be used for measuring the air speed of an aircraft by sequentially transmitting and receiving a laser light beam at a given pulse rate.
Document WO 2014/102175 thus describes a method and a system for determining the speed of an aircraft relative to air by using a laser anemometer device.
Furthermore, Document US 2010/0128252 describes a method and a system for optimizing the orientation of a laser anemometer.
Ultrasound anemometers also exist, such as the anemometer described in Document U.S. Pat. No. 4,031,756, that enable the air speed of an aircraft to be measured by transmitting and receiving ultrasound waves.
Furthermore, Document EP 2 799 890 describes a method and a system for determining the speed of an aircraft relative to the air on the basis in particular of the positions of other aircraft situated in its proximity.
Finally, Document FR 2 988 851 describes a method and a system for determining a credibility status for measurements from an incidence sensor of an aircraft.